He resorts to a secluded dwelling: the forest, the foot of a tree, a mountain, a glen, a hillside cave, a charnel ground, a forest thicket, a place out in the open, a heap of straw.

Often, the list is shortened as follows:

araññagato vā rukkhamūlagato vā suññāgāragato vā nisīdati...

Having gone to the forest or at the foot of a tree or in an empty room, he seats down...

In a few occasions, it is mentioned that brahmins or sages live in leaf huts in the forest (araññ·āyatane paṇṇa·kuṭīsu), as at DN 27, MN 93 and SN 11.9, or that the Buddha or other monks live in forest huts (arañña·kuṭika), as at SN 4.20, MN 125 or MN 136.

♦ Being a forest dweller (āraññika) was held in very high esteem by the Buddha and according to SN 15.13, it seems that he saw this practice, together with others, as very helpful for liberation:

These thirty bhikkhus from Pāvā are all forest dwellers, almsfood eaters, cast-off rags wearers, three-robes-only users [but] still fettered. What if I taught them the Dhamma in such a way that their minds would be liberated from their impurities through non-attachment [while sitting] on those very seats?

Bhante, for a long time I have been a forest dweller and have praised dwelling in the forest. I have been an almsfood eater and have praised eating almsfood. I have been a cast-off rags wearer and have praised wearing cast off rags. I have been a three-robes-only user and have praised using only three robes. I have been of few desires and have praised being of few desires. I have been content and have praised being content. I have been reclusive and have praised being reclusive. I have been lonely and have praised being lonely. I have been of aroused energy and have praised being of aroused energy.

Ananda, the new monks — those who have not long gone forth, who are newcomers in this Dhamma & Discipline — should be encouraged, exhorted, and established in these five things. Which five? (...) 'Come, friends, dwell in the forest. Resort to distant forest thicket lodgings.' Thus they should be encouraged, exhorted, & established in physical seclusion.

Endowed with five qualities, a bhikkhu practicing mindfulness of breathing will in no long time penetrate the unshakable [state]. Which five? (...) he is a forest dweller, [resorting to] distant lodgings

♦ The benefits of dwelling in the forest are mentioned in various places, such as AN 2.31:

And if, householders, heterodox spiritual wanderers ask you: 'And what are your reasons and with reference to what do you claim of those venerable ones: "Certainly, those venerable ones are devoid of lust or are practicing for the elimination of lust, they are devoid of aversion or are practicing for the elimination of aversion, they are devoid of delusion or are practicing for the elimination of delusion"?' If you are questioned thus, householders, you should answer to those heterodox spiritual wanderers: 'It is because those venerable ones resort to distant forest thicket lodgings, since there are no visible forms cognizable by the eye there such that, having seen them, they might delight in them; there are no sounds cognizable by the ear there such that, having heard them, they might delight in them; there are no smells cognizable by the nose there such that, having smelled them, they might delight in them; there are no tastes cognizable by the tongue there such that, having tasted them, they might delight in them; there are no bodily sensations cognizable by the body there such that, having felt them, they might delight in them; these, friends, are our reasons, it is with reference to this that we claim: "Certainly, those venerable ones are devoid of lust or are practicing for the elimination of lust, they are devoid of aversion or are practicing for the elimination of aversion, they are devoid of delusion or are practicing for the elimination of delusion".'

At MN 121, the perception of the forest that develops through forest dwelling is the basis for mental calm.

a bhikkhu, not directing his mind to the perception of villages, not directing his mind to the perception of humans, directs his mind to the oneness produced by the perception of the forest. His mind takes to that perception of the forest and grows confident, steady and settled.

At AN 6.42, the Buddha says he is pleased with a bhikkhu dwelling in the forest in three cases:

Nāgita, I see a forest dwelling bhikkhu sitting in the forest, dozing. It occurs to me: 'Soon this venerable one will dispel his sleepiness & fatigue and direct his mind only to the perception of the forest, {see MN 121 above} in a state of oneness.' And for this reason, I am pleased with that bhikkhu's dwelling in the forest.

Furthermore, Nāgita, I see a forest dwelling bhikkhu sitting in the forest, unconcentrated. It occurs to me: 'Soon this venerable one will concentrate his unconcentrated mind, or protect his concentrated mind.' And for this reason, I am pleased with that bhikkhu's dwelling in the forest.

Furthermore, Nāgita, I see a forest dwelling bhikkhu sitting in the forest, concentrated. It occurs to me: 'Soon this venerable one will liberate his unliberated mind, or protect his liberated mind.' And for this reason, I am pleased with that bhikkhu's dwelling in the forest.

♦ The fact that bhikkhus keep dwelling in the forest is very important, as stated at AN 7.33:

Furthermore, bhikkhus, in the future there will be monks desirous of fine lodgings. They, desirous of fine lodgings, will abandon the practice of dwelling at the foot of a tree, they will abandon distant forest thicket lodgings, they will move to villages, towns, and royal capitals and take up residence there. For the sake of lodgings they will engage in many kinds of wrong and inappropriate things.

It is difficult to endure distant forest thicket lodgings, it is difficult to practice seclusion, it is difficult to enjoy solitude. The forests, as it were, plunder the mind of a bhikkhu if he has not gained concentration.

The sutta then goes on to describe qualities that make forest dwelling difficult if not impossible, starting with lack of virtues, continuing with the five hindrances, and then miscellaneous qualities:

It is difficult to endure distant forest thicket lodgings, Upāli, it is difficult to practice seclusion, it is difficult to enjoy solitude. The forests, as it were, plunder the mind of a bhikkhu if he has not gained concentration. If anyone should say: 'Although I have not gained concentration, I will resort to distant forest thicket lodgings', it can be expected that he will either sink down or [merely] float [on the surface].

Imagine, Upāli, that there would be a large lake. A bull elephant would come, seven or eight cubits [high]. It would occur to him: 'What if, having entered this lake, I washed my ears by playing playfully, I washed my back by playing playfully; having washed my ears by playing playfully, having washed my back by playing playfully, having bathed, drunk and come out, I would go wherever I want.' He enters that lake, washes his ears by playing playfully, washes his back by playing playfully; having washed his ears by playing playfully, having washed his back by playing playfully, having bathed, drunk and come out, he goes wherever he wants. For what reason? Because his large body finds a footing in the depths.

Then a hare or a cat would come. It would occur to it: 'Why should a bull elephant be different from me? What if, having entered this lake, I washed my ears by playing playfully, I washed my back by playing playfully; having washed my ears by playing playfully, having washed my back by playing playfully, having bathed, drunk and come out, I would go wherever I want.' It would inconsiderately and thoughtlessly jump into the lake. It can be expected that it will either sink down or [merely] float [on the surface]. For what reason? Because its small body doesn't find a footing in the depths. In the same way, Upāli, if anyone should say: 'Although I have not gained concentration, I will resort to distant forest thicket lodgings', it can be expected that he will either sink down or [merely] float [on the surface].

Endowed with [any of] four qualities, a monk isn't fit to stay in isolated forest & wilderness dwellings. Which four? [He is endowed] with thoughts of sensuality, with thoughts of ill will, with thoughts of harmfulness, and he is a person of weak discernment, dull, a drooling idiot. Endowed with [any of] these four qualities, a monk isn't fit to stay in isolated forest & wilderness dwellings.

Endowed with four qualities, a monk is fit to stay in isolated forest & wilderness dwellings. Which four? [He is endowed] with thoughts of renunciation, with thoughts of non-ill will, with thoughts of harmlessness, and he is a discerning person, not dull, not a drooling idiot. Endowed with these four qualities, a monk is fit to stay in isolated forest & wilderness dwellings.

"Migajala, there are forms cognizable via the eye — agreeable, pleasing, charming, endearing, fostering desire, enticing — and a monk relishes them, welcomes them, & remains fastened to them. As he relishes them, welcomes them, & remains fastened to them, delight arises. There being delight, he is impassioned. Being impassioned, he is fettered. A monk joined with the fetter of delight is said to be a person living with a companion. There are sounds cognizable via the ear... aromas cognizable via the nose... flavors cognizable via the tongue... tactile sensations cognizable via the body... ideas cognizable via the intellect — agreeable, pleasing, charming, endearing, fostering desire, enticing — and a monk relishes them, welcomes them, & remains fastened to them. As he relishes them, welcomes them, & remains fastened to them, delight arises. There being delight, he is impassioned. Being impassioned, he is fettered. A monk joined with the fetter of delight is said to be a person living with a companion. A person living in this way — even if he frequents isolated forest & wilderness dwellings, with an unpopulated atmosphere, lying far from humanity, appropriate for seclusion — is still said to be living with a companion. Why is that? Because the craving that is his companion has not been abandoned by him. Thus he is said to be a person living with a companion.